While you ARE technically correct, the alternative in the wild is progressive taxation. That makes flat taxation a de facto regressive tax, as it will be more regressive than the current taxation regime.
I think this whole tax debate misses an important point. Taxes are viewed as revenue in terms of budget and there is a business tendency to make more revenue to cover expenses. If you stop seeing Tax as a major source of revenue and start having Govt invest money in different ways to make money other than just taxes, we can start actually having a real debate around what is a fair tax rate for govt to cover their expenses and figure out a way to make it progressive.